Optical split

ABSTRACT

An optical slit comprises two blades  40,42  which define a slit between them, each blade being independently movable. This enables both the slit position and the slit width to be adjusted. The slit may be aligned with the centre of a light beam by aiming the light beam at a detector, traversing at least one edge of the slit across the beam path, measuring the intensity of transmitted light at the detector for each position of the slit, and feeding back a signal which adjusts the slit position for maximum light throughput. The width on the optical slit may be selected by placing the slit in the path of the light beam and measuring the light transmitted at the detector, calculating the percentage of light transmitted for that slit width and feeding back a signal which adjusts slit width to obtain the desired amount of light throughput.

[0001] This invention relates to an optical slit, for example for use ina spectroscopy system.

[0002] In our earlier European Patent Specification EP 0543578, a sampleis irradiated with monochromatic light from a laser and scattered lightis analysed in order to select a particular line of the resulting Ramanspectrum. The analysis may be performed by a dispersive device such as adiffractive grating or it may be performed using a non-dispersivetunable filter. The resulting Raman scattered light may be focused ontoa charge-coupled device (CCD) which is a two-dimensional photodetectorarray.

[0003] European Patent No. 0542962 discloses a method of spectroscopy asabove in which one-dimensional confocality is achieved by use of aspatial filter comprising an optical slit.

[0004] A first aspect of the invention provides an optical slit devicecomprising two blades defining a slit between them wherein each blademay move independently of the other.

[0005] Preferably the blades are motorised.

[0006] A second aspect of the invention provides a method for aligningan optical slit with the centre of a light beam in a spectroscopy systemcomprises:

[0007] aiming the light beam at a detector;

[0008] traversing at least one edge of an optical slit across the beampath;

[0009] detecting the intensity of transmitted light at the detector at aplurality of positions of the at least one edge of the optical slit;

[0010] and feeding back a signal which adjusts the slit position.

[0011] A third aspect of the invention provides a method for setting thewidth on an optical slit in a spectroscopy system comprises:

[0012] aiming a light beam at a detector;

[0013] placing the slit in the path of the light beam;

[0014] detecting the intensity of transmitted light at the detector forthat slit width;

[0015] calculating the ratio of transmitted light for that slit widthcompared to the amount of transmitted light with no slit;

[0016] and feeding back a signal which adjusts slit width to obtain thedesired amount of light throughput.

[0017] Preferred embodiments of the invention will now be described byway of example, with reference to the accompanying drawings wherein:

[0018]FIG. 1 is a schematic diagram of a prior Raman system;

[0019]FIG. 2 is an isometric view of the spatial filter;

[0020]FIGS. 3A and 3B are side views of flexure units used in FIG. 2;

[0021]FIG. 4 is a plan view of an optical slit and light beam;

[0022]FIG. 5 shows a typical image intensity profile on a CCD;

[0023]FIG. 6 shows a typical plot of light intensity at the CCD as twoedges of a slit are traversed together;

[0024]FIG. 7 shows a typical plot of light intensity at the CCD as twoedges of the slits are traversed independently;

[0025]FIG. 8 shows a plot of light intensity at the CCD as the slit istraversed across a light beam when the slit width is less than the beamdiameter;

[0026]FIG. 9 shows a plot of light intensity at the CCD as the slit istraversed across a light beam when the slit width is greater than thebeam diameter;

[0027]FIG. 10 shows a block diagram of the method of finding the beamcentre;

[0028]FIG. 11 shows a block diagram of an alternative method of findingthe beam centre;

[0029]FIG. 12 illustrates the method of FIG. 11 in which both bladesdefining the slit are moved; and

[0030]FIG. 13 illustrates the method of FIG. 11 in which only one of thetwo blades defining the slit is moved.

[0031] Referring to the drawings, the previously known arrangement shownin FIG. 1 has a dichroic filter 12 arranged at 45° to an incoming laserbeam 10. It could instead be arranged at a low angle to the beam, asexplained in EP 0543578. The incoming beam is reflected by the dichroicfilter 12 and then focused by an objective lens 14 onto a point on thesample 16. Light scattered from the point on the sample is collected bythe objective lens 14, collimated into a parallel beam and passedthrough the dichroic filter 12. The dichroic filter 12 rejects theRayleigh scattered light having the same frequency as the input beam buttransmits the Raman scattered light. The Raman scattered light isbrought to a tight focus by a lens 18 at a spatial filter comprising ascreen 22 with a slit 24. Light scattered from the focal point of theobjective lens passes through the slit 24. Most of the light scatteredfrom behind or in front of the focal point is blocked by the screen 22as it does not come to a focus at the slit. Thus the slit confersone-dimensional confocality to the scattered light. Light passingthrough the slit is collimated by a lens 26 to a parallel beam andpasses through the Raman analyser 28. The analyser may produce aspectrum (i.e. when a diffraction grating is used) or select light fromjust one frequency (e.g. by using a tunable non-dispersive filter). Alens 30 brings the analysed light to a tight focus on the photodetector32 which may be a charge-coupled device (CCD).

[0032] In our co-pending International application PCT/GB02/01039 filed14th Mar. 2002 the slit 22 is required to move from side to side toenable scanning of the sample. An adjustable slit width would allow theslit to be adjusted between confocal and non-confocal settings.

[0033] A preferred embodiment of the spatial filter is shown in FIG. 2in which two movable blades 40,42 define a slit between them. Each blade40,42 is mounted on an anvil 46,48 which in turn is mounted on a flexureunit shown in FIG. 3. Each flexure unit shown in FIG. 3A, comprises twospaced-apart parallel plates 60,62 one above the other, connected by twoplanar springs 64,66 which join opposite edges of the upper plate 62 toopposite edges of the lower plate 60 to form a box shape. Thisarrangement allows the upper plate 62 and thus the anvil 46,48 to movein a first horizontal direction relative to the lower plate as seen inFIG. 3B (i.e. in the direction of opening and closing the slit, parallelto arrow X in FIG. 2), whilst the upper and lower plates remainparallel. Movement of the upper plate in a second directionperpendicular to the first direction is constrained.

[0034] The movement of each anvil is controlled by a motor 50,52. Eachmotor is mounted in the centre of a planar spring 54. Each spring hascut-outs 56 which allow movement of the motor parallel to the motoraxis, yet constrains any rotational movement about this axis. The springalso allows some lateral movement and tilting to overcome any smallalignment errors of the motors.

[0035] As the upper plate 62 of the flexure unit moves from side to sideits distance relative to the lower plate 60 will vary due to the bendingof the planar springs 64,66 connecting them, seen in FIGS. 3A and 3B.This variation causes the distances between the slit 24 and the sample16 and between the slit 24 and the photodetector 32 to change withdifferent slit positions. Therefore it is necessary to include an erroradjustment in the analysed data dependent upon slit position.

[0036] The two blades 40,42 may either move together or independently.To move together one motor is used to move one of the anvils in thedesired direction, this anvil will push the other one along with theslit width remaining constant, i.e. at its minimum width. For example,motor 52 is used to move anvil 48 in direction X. Anvils 48 and 46 haveinterlocking shapes such that protruding portion 58 of anvil 48 pushesagainst a recess portion 59 of anvil 46, resulting in anvil 48 pushinganvil 46. A spacer element 62 is provided on the recess portion 59 ofanvil 46 adjacent the protruding portion 58 of anvil 46. This spacerelement 62 ensures the anvils 46 and 48 are a constant distance apartduring this operation and thus the slit width is constant at its minimumwidth.

[0037] Alternatively both motors may be used to move the two anvils andthus the two blades independently. This permits both the width and theposition of the slit to be varied.

[0038] This arrangement is also suitable for use in aligning the slitwith a light beam in a spectrometer. Confocal and small laser spotrequirements for Raman microscopy require very fine adjustment of thealignment of the laser beam onto a traditional spectrometer slit.Alignment of the laser onto the slit is subject to long-term stabilityproblems and generally needs to be adjusted. Use of the motorised slitsdescribed in the above embodiment enables the position of the centre ofthe opening of the slit to be varied.

[0039]FIG. 4 shows two blades 40,42 defining a slit and a beam 70 in aspectrometer. It is desirable for the position of the centre of theopening 72 of the optical slit to correspond with the beam centre 74 ofthe laser. Thus the beam position must be calculated in order to adjustthe centre of opening position accordingly.

[0040] In a first method the slit width between blades 40,42 is keptfixed, for example at 50 μm and the slit is traversed across the beampath. The traversal of the slit may be step-wise or continuous. Duringthis manoeuvre, the intensity of the transmitted light is measured atthe CCD for each position of the slit.

[0041]FIG. 5 shows a small portion of the CCD 32 comprising atwo-dimensional array of pixels 76. The values shown on the CCDcorrespond to the light intensity of the image at each pixel. Thesevalues will differ for each slit position.

[0042] The total value of the light intensity measured at the CCD foreach slit position is plotted and curve fitted on a graph shown in FIG.8. The highest value of the total intensity 80 corresponds to theposition of the beam centre (i.e. when the position of the centre of theopening of the optical slit was aligned with the beam centre). Feedbackfrom the detector is used to drive the blades to their calculatedpositions, derived from the light intensity measurements at thedetector. The slit has thus now been moved to a position aligned withthe beam centre.

[0043] This method is shown in summary in FIG. 10.

[0044] In a second method of determining the centre beam position 74,each blade 40,42 of the slit is traversed independently across the beam70. As in the previous method the total intensity of transmitted lightis measured for each slit position. Firstly blade 40 is moved across thebeam to produce curve A seen in FIG. 9, then blade 42 is traversedacross the beam to produce curve B. The data from both thesemeasurements are combined to determine the position of the beam centre82. The slit may then be moved so that the position of the centre of theopening of the slit 72 is aligned with the beam centre 74.

[0045] In an alternative method of determining the beam centre, theblades may be moved together or separately in bisections to reduce thenumber of measurements required, thus reducing the uncertainty by afactor of two for each step.

[0046] The slit is placed between the light source and the photodetectorand the total value for the light intensity at the photodetector ismeasured for each position of the blades. The slit is set at an initialwidth 80 and the total light intensity at the photodetector is measured82. It is determined whether the light transmitted to the photodetectorfor this slit position is over a certain threshold 84, for example 50%.If the transmitted light is over the threshold, the slit width isbisected 86. If the transmitted light is under the threshold, the slitis moved to the other half of the previous position 88. This process isrepeated, each time halving the slit width until the centre of the beamis found. The slit is then positioned at the centre of the beam.

[0047]FIG. 12 illustrates this method in which both blades defining theoptical slit are moved together. In step 2, the blades 40,42 are adistance x apart. The detected light intensity is greater than 50% andso the slit width is bisected to x/2, as shown in step 2. In step 2 thedetected light intensity is less than 50% so the slit width remains thesame but the slit position is moved to the other half of the previousposition as shown in step 3. In step 3 the detected light intensity isover 50% so the slit width is bisected to x/4 as shown in step 4. Instep 4 the detected light intensity is less than 50% and so the slitwidth remains at x/4 but the position is moved to the other half of theprevious position as shown in step 5. In step 6 the detected lightintensity is greater than 50% and so the slit width is bisected. Oncethe minimum slit width, a, has been reached as in step 7, the slit isaligned with the centre of the beam.

[0048] Thus the slit width is halved on every step, or every other step.The highest possible number of steps to find the centre is two times thepower of two from maximum to minimum slit width.

[0049]FIG. 13 illustrates the method in which one blade 42 remainsstationary whilst the other blade 40 moves to vary the slit position.Each time the detected light intensity is greater than 50%, the blade 40is moved towards the blade 42 by half the current slit width as shown insteps 1-3. If the light intensity is less than 50%, the blade 40 ismoved away from the blade 42 by half the current slit width as shown instep 5. The centre of the beam is thus found and the minimum step sizeof the slit is reached. In this method the step size is halved everystep and the maximum number of steps to find the beam centre is thus thepower of two from maximum to minimum slit width.

[0050] This method has the advantage that it is faster than thepreviously described methods in which the beam centre is found bytraversing the slit across the beam. For example a 1 μm slit may bealigned with the centre of the 2 mm beam in just 11 steps by thismethod.

[0051] The motorised slit arrangement described above also enables theslit width to be varied. This is useful for adjusting between confocaland non-confocal settings. The width of the slit is indicated by thepercentage of light transmitted to the CCD. For example a confocalsetting may give 80% light transmission whilst a non-confocal settingmay give 98% transmission. A desired slit width may therefore be set byadjusting the width of the slit until the corresponding percentage oflight is transmitted to the CCD. The effect of different slit widths canbe seen in FIGS. 8 and 9. Both Figures are plots of total lightintensity measured at the CCD for different slit positions (as in FIG.6). FIG. 8 is the case where slit width is less than the beam diameter(i.e. confocal) and FIG. 9 is the case where slit width is larger thanthe beam diameter (i.e. non-confocal).

[0052] The motorised slit arrangement is not restricted to the usedescribed above and is suitable for various other uses, for exampleshaping beams, imaging, beam aperturing, beam profiling, measurementsand resolution.

[0053] Furthermore the motorised slit arrangement is suitable for use ina variety of optical systems, for example spectrographs, monochromators,Fourier filtering and modulation transfer functions.

1. An optical slit device comprising two blades defining a slit betweenthem wherein each blade may move independently of the other.
 2. Anoptical slit device according to claim 1 wherein the blades aremotorised.
 3. An optical slit device according to any preceding claimwherein the two blades are each mounted on a flexure unit.
 4. An opticalslit device according to claim 2 wherein the motor driving each blade ismounted on a spring to allow movement parallel to the axis of the motoryet constrains rotational movement about said axis.
 5. An optical slitdevice according to any preceding claim wherein the slit is at adifferent distance from the photodetector and sample for each slitposition and wherein an error adjustment is included in the analyseddata for different slit positions to correct for these differentdistances.
 6. An optical slit device according to any preceding claimwherein the blades may be moved together.
 7. An optical slit deviceaccording to claim 6 wherein a motor is used to move one of the bladeswhich in turn pushes the other blade.
 8. An optical slit deviceaccording to claim 7 wherein a spacer is provided to keep the blades ata minimum width.
 9. An optical slit device according to any of claims2-8 wherein: a controller is provided to drive the movement of at leastone blade; such than when the optical slit device is positioned in abeam path between a light source and a detector, the controller maydrive at least one of the blades across the beam path, such that theintensity of transmitted light at the detector is detected for aplurality of positions of the at least one blade; and wherein thecontroller feeds back a signal from the detector to adjust the positionof the at least one blade.
 10. An optical slit device according to claim7 wherein the controller drives both blades across the beam pathtogether, at a constant distance apart.
 11. An optical slit deviceaccording to claim 9 wherein: the controller drives a first blade of thetwo blades across the beam path whilst the intensity of transmittedlight at the detector is detected for each position of the first blade;the controller drives a second blade of the two blades across the beampath whilst the intensity of transmitted light at the detector isdetected for each position of the second blade; and wherein thecontroller combines the data from these two steps.
 12. An optical slitdevice according to claim 9 wherein: the controller determines whetherthe intensity of transmitted light is above a threshold for eachposition of the at least one blade; such that if the intensity of thetransmitted light is above the threshold, the controller moves the atleast one blade in order to sub-divide the slit width.
 13. An opticalslit device according to claim 12 wherein: the controller drives bothblades across the beam; such that if the intensity of the transmittedlight is above the threshold, the controller drives the blades in orderto sub-divide the slit width and wherein if the intensity of thetransmitted light is below the threshold, the controller drives theblades in order to move the slit within its previous position whilst theslit width remains the same.
 14. An optical slit device according toclaim 9 wherein: the controller drives a first blade of the two bladesacross the beam whilst the second blade of the two blades remainsstationary; such that if the intensity of the transmitted light is abovethe threshold, the controller drives the first blade in order toposition it towards the second blade, and wherein if the intensity ofthe transmitted light is below the threshold, the controller drives thefirst blade away from the second blade to increase the slit width. 15.An optical slit device according to any of claims 12-14 wherein the slitis sub-divided into substantially half its previous width.
 16. Anoptical slit device according to any of claims 12-14 wherein thethreshold is substantially 50% of the intensity of transmitted lightwith the optical slit removed.
 17. An optical slit device according toany of claims 2-8 wherein: a controller is provided to drive themovement of at least one blade; such that when the optical slit deviceis positioned in a beam path between a light source and a detector, theratio of transmitted light for that slit width compared to the amount oftransmitted light with no slit is calculated; and wherein a signal isfed back to the controller to move the at least one blade to adjust theslit width to obtain the desired amount of light throughput.
 18. Amethod for aligning an optical slit with the centre of a light beam in aspectroscopy system comprises: aiming the light beam at a detector;traversing at least one edge of an optical slit across the beam path;detecting the intensity of transmitted light at the detector at aplurality of positions of the at least one edge of the optical slit; andfeeding back a signal which adjusts the slit position.
 19. A method foraligning an optical slit according to claim 18 wherein the slit positionis adjusted for maximum light throughput at a given slit width.
 20. Amethod for aligning an optical slit according to claim 18 wherein twoedges of the optical slit are traversed across the beam path togetherwith the edges at a constant distant apart.
 21. A method for aligning anoptical slit according to claim 18 wherein: a first edge of an opticalslit is traversed across the beam path and the intensity of transmittedlight at the detector is detected for each position of the first edge ofthe optical slit; a second edge of an optical slit is traversed acrossthe beam path and the intensity of transmitted light at the detector isdetected for each position of the second edge of the optical slit; andwherein the data from these measurements are combined.
 22. A method foraligning an optical slit according to claim 18 wherein: it is determinedwhether the intensity of transmitted light is above a threshold for eachposition of the at least one edge of the optical slit; such that if theintensity of the transmitted light is above the threshold, the at leastone edge of the optical slit is moved in order to sub-divide the slitwidth.
 23. A method for aligning an optical slit according to claim 22wherein: both edges of the optical slit are traversed across the beam;such that if the intensity of the transmitted light is above thethreshold, the edges of the optical slit are moved in order tosub-divide the slit width and wherein if the intensity of thetransmitted light is below the threshold, the edges of the optical slitare moved in order that position of the slit is moved within theprevious position whilst the slit width remains the same.
 24. A methodof aligning an optical slit according to claim 22 wherein: a first edgeof the optical slit is traversed across the beam whilst the second edgeremains stationary; such that if the intensity of the transmitted lightis above the threshold, the first edge of the optical slit is moved inorder to position it closer to the second edge, and wherein if theintensity of the transmitted light is below the threshold, the firstedge is moved to position it further away from the second edge toincrease the slit width.
 25. A method of aligning an optical slitaccording to any of claims 22-24 wherein the slit is sub-divided intosubstantially half its previous width.
 26. A method of aligning anoptical slit according to any of claims 22-24 wherein the threshold issubstantially 50% of the intensity of transmitted light with the opticalslit removed.
 27. A method for setting the width on an optical slit in aspectroscopy system comprises: aiming a light beam at a detector;placing the slit in the path of the light beam; detecting the intensityof transmitted light at the detector for that slit width; calculatingthe ratio of transmitted light for that slit width compared to theamount of transmitted light with no slit; and feeding back a signalwhich adjusts slit width to obtain the desired amount of lightthroughput.